1. Field of the Invention
This invention relates to a method of forming blisters which are useful in the blister packing technique used for packing foods, drugs, etc.
2. Description of the Related Art
Recently, polypropylene web materials in the form of sheets, films, etc. have come to be used frequently as a packing material for blister packing, and in particular, for PTP (Push Through Packing) which is mainly used for packing drugs. When using such polypropylene web materials, it is the general practice to adopt a "preheater hot-plate-contact compressed-air system" as the technique for forming blisters. This "preheater hot-plate-contact compressed-air system" employs a forming device which has in its forming section a cavity section having cavities and a compressed-air box facing this cavity section and equipped with plugs. A preheated web material is placed between the cavity section and the compressed-air box, blisters being formed on the web material by means of the plugs and compressed air. In this system, a web material is heated by placing it between hot plates. If the heating temperature is relatively high, various inconveniences may be involved, such as fusion of the web material to the surface of the hot plate, the generation of creases, elongation of the web material due to tension during feeding, and transfer of the surface roughness etc. of the hot plate to the surface of the web material, which will lead to various types of defective formation as well as insufficient transparency of the blisters. Conversely, if the heating temperature is relatively low, the form reproducibility of the forming section will deteriorate. By form reproducibility is meant here the ability to form blisters into a configuration identical to that of the cavities in the forming section. The suitable temperature range is very narrow and therefore, it is necessary to make the hot plate having a high accuracy in a temperature and no temperature difference as to a web material in the plate direction. Besides, in order to stabilize the formation, there is a limit to the type of web material that can be used. That is, the material must have a low softening point and no tendency to melt. The material must be of high uniformity and of high quality. It must exhibit high accuracy in wall thickness, involving no flapping or snaking. In order to overcome these difficulties, a technique (Japanese Patent Laid-Open No. 58-126117) has been proposed according to which cavities are formed in the forming section of a rotary forming drum, suction holes being provided in the non-forming area of the drum, a web material being closely contacted around the peripheral surface of the drum, and the web material fed from the rotary forming drum being heated while receiving a low tension of 0.1 kg/cm from a dancer roll. Since the heating and forming are conducted on the rotary forming drum, it is capable of continuously performing the thermoforming of web materials and that at the same time, since the web material can be wound around the rotary forming drum with a satisfactory tightness, the forming operation can be performed in a stable manner as long as a proper web material is selected by the technique mentioned above.
The problem with the above prior art technique is that the tension applied to the web material by means of the dancer roll is as low as about 0.1 kg/cm, which can lead to defective formation in part since the web material may not adhere to the rotary forming drum in a satisfactory manner, depending on the physical properties (heat expansion, heat contraction, etc.) and other properties (wall-thickness accuracy, flapping, snaking, etc.) of the material.
In some cases, the configuration of the rotary forming drum used for blister formation is made polygonal, e.g., octagonal, so that it may be well suited for the formation of relatively long blisters.
However, a polygonal drum involves the problem that gaps are left between the drum peripheral surface and the web material would around it, so that those sections where no blisters are to be formed are also heated, resulting in creases being generated all over. As a result, the blisters become hard to seal with cover material after they are filled with drugs.